1. Field of the Invention
The present invention relates to electronic circuits for powering a variable-optic electronic ophthalmic lens or other similar device, and more particularly, to H-Bridge type switching circuits for a lens driver configured to control optic elements in a variable-optic electronic ophthalmic lens.
2. Discussion of the Related Art
As electronic devices continue to be miniaturized, it is becoming increasingly more likely to create wearable or embeddable microelectronic devices for a variety of uses. Such uses may include monitoring aspects of body chemistry, administering controlled dosages of medications or therapeutic agents via various mechanisms, including automatically, in response to measurements, or in response to external control signals, and augmenting the performance of organs or tissues. Examples of such devices include glucose infusion pumps, pacemakers, defibrillators, ventricular assist devices and neurostimulators. A new, particularly useful field of application is in ophthalmic wearable lenses and contact lenses. For example, a wearable lens may incorporate a lens assembly having an electronically adjustable focus to augment or enhance performance of the eye. In another example, either with or without adjustable focus, a wearable contact lens may incorporate electronic sensors to detect concentrations of particular chemicals in the precorneal (tear) film. The use of embedded electronics in a lens assembly introduces a potential requirement for communication with the electronics, for a method of powering and/or re-energizing the electronics including power control or power management circuitry, for interconnecting the electronics, for internal and external sensing and/or monitoring, and for control of the electronics and the overall function of the lens.
The human eye has the ability to discern millions of colors, adjust easily to shifting light conditions, and transmit signals or information to the brain at a rate exceeding that of a high-speed internet connection. Lenses, such as contact lenses and intraocular lenses, currently are utilized to correct vision defects such as myopia (nearsightedness), hyperopia (farsightedness), presbyopia and astigmatism. However, properly designed lenses incorporating additional components may be utilized to enhance vision as well as to correct vision defects.
Contact lenses may be utilized to correct myopia, hyperopia, astigmatism as well as other visual acuity defects. Contact lenses may also be utilized to enhance the natural appearance of the wearer's eyes. Contact lenses or “contacts” are simply lenses placed on the anterior surface of the eye. Contact lenses are considered medical devices and may be worn to correct vision and/or for cosmetic or other therapeutic reasons. Contact lenses have been utilized commercially to improve vision since the 1950s. Early contact lenses were made or fabricated from hard materials, were relatively expensive and fragile. In addition, these early contact lenses were fabricated from materials that did not allow sufficient oxygen transmission through the contact lens to the conjunctiva and cornea which potentially could cause a number of adverse clinical effects. Although these contact lenses are still utilized, they are not suitable for all patients due to their poor initial comfort. Later developments in the field gave rise to soft contact lenses, based upon hydrogels, which are extremely popular and widely utilized today. Specifically, silicone hydrogel contact lenses that are available today combine the benefit of silicone, which has extremely high oxygen permeability, with the proven comfort and clinical performance of hydrogels. Essentially, these silicone hydrogel based contact lenses have higher oxygen permeability and are generally more comfortable to wear than the contact lenses made of the earlier hard materials.
Conventional contact lenses are polymeric structures with specific shapes to correct various vision problems as briefly set forth above. To achieve enhanced functionality, various circuits and components have to be integrated into these polymeric structures. For example, control circuits, microprocessors, communication devices, power supplies, sensors, actuators, light-emitting diodes, and miniature antennas may be integrated into contact lenses via custom-built optoelectronic components to not only correct vision, but to enhance vision as well as provide additional functionality as is explained herein. Electronic and/or powered contract lenses may be designed to provide enhanced vision via zoom-in and zoom-out capabilities, or just simply modifying the refractive capabilities of the lenses. Electronic and/or powered contact lenses may be designed to enhance color and resolution, to display textural information, to translate speech into captions in real time, to offer visual cues from a navigation system, and to provide image processing and internet access. The lenses may be designed to allow the wearer to see in low-light conditions. The properly designed electronics and/or arrangement of electronics on lenses may allow for projecting an image onto the retina, for example, without a variable-focus optic lens, provide novelty image displays and even provide wakeup alerts. Alternately, or in addition to any of these functions or similar functions, the contact lenses may incorporate components for the noninvasive monitoring of the wearer's biomarkers and health indicators. For example, sensors built into the lenses may allow a diabetic patient to keep tabs on blood sugar levels by analyzing components of the tear film without the need for drawing blood. In addition, an appropriately configured lens may incorporate sensors for monitoring cholesterol, sodium, and potassium levels, as well as other biological markers. This, coupled with a wireless data transmitter, could allow a physician to have almost immediate access to a patient's blood chemistry without the need for the patient to waste time getting to a laboratory and having blood drawn. In addition, sensors built into the lenses may be utilized to detect light incident on the eye to compensate for ambient light conditions or for use in determining blink patterns.
The proper combination of devices could yield potentially unlimited functionality; however, there are a number of difficulties associated with the incorporation of extra components on a piece of optical-grade polymer. In general, it is difficult to manufacture such components directly on the lens for a number of reasons, as well as mounting and interconnecting planar devices on a non-planar surface. It is also difficult to manufacture to scale. The components to be placed on or in the lens need to be miniaturized and integrated onto just 1.5 square centimeters of a transparent polymer while protecting the components from the liquid environment on the eye. It is also difficult to make a contact lens comfortable and safe for the wearer with the added thickness of additional components.
Given the area and volume constraints of an ophthalmic device such as a contact lens, and the environment in which it is to be utilized, the physical realization of the device must overcome a number of problems, including mounting and interconnecting a number of electronic components on a non-planar surface, the bulk of which comprises optic plastic. Accordingly, there exists a need for providing a mechanically and electrically robust electronic contact lens.
As these are powered lenses, energy or more particularly current consumption to run the electronics is a concern given battery technology on the scale for an ophthalmic lens. In addition to normal current consumption, powered devices or systems of this nature generally require standby current reserves, precise voltage control and switching capabilities to ensure operation over a potentially wide range of operating parameters, and burst consumption, for example, up to eighteen (18) hours on a single charge, after potentially remaining idle for years.
Vision correction, and potentially vision enhancement, is typically achieved in spectacle lenses, contact lenses, intraocular lenses (IOL's) and other ophthalmic devices through static optics. For example, spectacle lenses or contact lenses to treat myopia (nearsightedness) comprise lenses with spherical power to correct focus onto the retina caused by defects in the cornea and/or lens. Bifocal corrective lenses may contain an inset lens of a different power than the main lens. More advanced designs use gradient, zone, or other schemes to vary corrective power over the lens. However, because these lenses are optically static, they do not match the human eye's natural response which is a variable-focus action accomplished by varying the optical power of the eye's crystalline lens. In presbyopic individuals, the eye's natural ability to accommodate with different focal lengths is greatly reduced leading to a loss of function and annoyance. Recent advancements in the field have included spectacle lenses and even IOL's with some dynamic accommodation, for example, electronic spectacle lenses or IOL's connected to the eye's zonules to achieve a limited amount of optical power change. These existing systems are limited by only covering a small range of add power, perhaps only +1 diopter, requiring spectacle lenses to be worn, requiring surgery to implant an IOL, and other drawbacks.
There are several types of electronically variable lens technologies, including liquid crystal, electro-active polymer, electro-mechanical, variable fluid, and liquid meniscus lenses. Such electronically variable lenses require an actuator, and an electronic device to alter the focal length of the lens. For example, in a liquid meniscus or electro-active polymer lens, an applied voltage and/or current from an actuator modulates physical parameters of the lens to vary the focal length. Both variable lenses and their actuators, also known as lens drivers, are commercially available for various applications such as smartphone cameras and industrial applications. Suitable lenses and actuators do not exist for ophthalmic devices such as contact lenses and IOL's.
Electrical or powered lenses typically require higher voltage than what is immediately available from a battery. For example, a powered lens may require sixty (60) volts to reach the maximum change of focal length but typical batteries output less than four (4) volts. Typical lens drivers include a voltage multiplier circuit to achieve high output voltage from a low-voltage source, many designs of which are known in the art. A voltage multiplier is essentially a voltage and current conversion device, similar in principal to that of an electric transformer with mismatched primary-to-secondary ratios. Whereas a transformer operates on alternating current, a voltage multiplier operates from a direct current (DC) source such as a battery. A voltage multiplier may comprise a charge pump, a circuit type widely known in the electronics art.
Lens drivers which are presently available have many disadvantages which make them unsuitable for use in ophthalmic devices such as contact lenses and IOL's. Current consumption of typical lens drivers is on the order of approximately one (1) to more than one hundred (100) milliamps. While this is acceptable current consumption for a robotic manufacturing system with access to main line power or even a camera or smartphone with a relatively large battery, it is far too much current for a power source in an ophthalmic device. Such power sources, implemented as batteries, energy harvesters, and/or capacitors, are typically limited to current of perhaps thirty (30) microamps or less. Both the active current consumption, the current drawn by the lens driver when activating the powered lens, and the standby current consumption, the current drawn when the lens driver is not driving the powered lens, are critical parameters for an ophthalmic device.
Typical electronically variable lenses and their lens drivers are designed for applications and not optimized for ophthalmic device usage. For example, some lenses are continuously variable over a range of focal lengths from millimeters to infinity, some thirty (30) or more diopters. Commercial lenses and drivers must change focal length very quickly, perhaps within less than one hundred (100) milliseconds. Ophthalmic lenses may only need to change focus in one (1) or two (2) seconds, the time typically required for the natural eye to change focal distance, as is known in the art. With liquid meniscus lenses, even with the lens voltage charged in 100 milliseconds, the lens itself would take 1 to 2 seconds to change focus; however, with liquid crystal optics, a lens activation of 100 milliseconds is achievable and the optics will respond more quickly. Typical lens and driver systems intended for commercial and manufacturing applications must last for many years in operation and undergo wide changes in focal length many times per day. In contrast, some ophthalmic devices such as contact lenses may be disposable and only used for eighteen (18) hours.
Typical lens drivers are implemented with discrete electronics or integrated circuits (IC's). Even when implemented as IC's, lens drivers may require external components such as capacitors, and the physical die size of the lens driver may be two (2) square millimeters or more at a thickness of hundreds of microns and thus still a challenge.
Electrically variable lenses are typically activated with a voltage of ten (10) to sixty (60) volts. Thus, lens drivers for these devices must output a high voltage sufficient to activate the powered lens. Lens drivers may be programmable to change the output voltage thereby modulating the optical power of the powered lens.
Due to requirements for speed, reliability, and precise modulation of optical power over a large range of focal distances, typical lens drivers for liquid meniscus lenses utilize an alternating current (AC) driver. Such an AC driver rapidly switches the bias applied to the lens between positive and negative, perhaps at a one kilohertz (1 kHz) rate. Other types of optics may require lower frequencies, for example, 25 to 50 Hz. This drive method provides benefits for existing commercial applications, but also greatly increases current consumption from the alternative direct current (DC) drive method. The liquid meniscus lens may be modeled as a capacitor, and as such the energy required to charge the capacitor is ½×C×V2 where C is the lens capacitance and V is the applied voltage. Liquid lens capacitance is approximately two hundred picofarads (200 pF). It is apparent that a large amount of power is provided and consumed by a typical high-voltage lens driver since the lens capacitance must be charged at a fast rate.
Accordingly, there exists a need for a lens driver for a powered ophthalmic lens that is optimized for low cost, long-term reliable service, safety, size, and speed while providing the requisite power to drive a variable-focus optic.